Plutonium etching process



United States Patent 3,375,149 PLUTONIUM ETQHBNG PROCESS Morris Kolodney, New York, N.Y., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Apr. 28, 1948, Ser. No. 23,865 4 Claims. (Cl. 156-18) This invention relates to a method of treating metals and more particularly to a method of treating plutonium metal compositions.

Plutonium is a very active metal which readily oxidizes in air, with the resulting oxide tending to flake off the plutonium surface as a powder. This is very undesirable because it means the loss of valuable plutonium, because these oxides are both radioactive and toxic, and because corrosion means that the plutonium objects are not dimensionally stable. It is, therefore, desirable that the plutonium metal compositions be coated with a smooth, uniform, adherent coating which will prevent the oxidation of the plutonium in air. In addition, such coatings may be designed to absorb the alpha radiation of the plutonium and to make it possible to handle and store the plutonium with greater safety. It is, therefore, desirable that methods be known for caching plutonium metal compositions so that they may readily be coated.

It is also desirable that etching may be carried out with solutions which may be readily handled in a closed system and that the etching process be as simple as possible.

It is, therefore, the primary object of this invention to provide a method for simply, quickly, and efficiently treating plutonium metal compositions.

It is a further object of this invention to provide a convenient method for etching plutonium metal compositions in a closed system.

It is a still further object of this invention to provide a method for etching plutonium metal compositions so that they may be efliciently covered with a smooth, uniform coating.

Still further objects and advantages of this invention will be evident from the following description and claims.

The objects of this invention are achieved by a process which comprises immersing the plutonium metal composition in concentrated orthophosphoric acid. The orthophosphoric acid etches the plutonium metal composition so that it may be readily coated. At times, however, a brown complex forms on the surfaces of the plutonium metal composition, which may be removed by vigorously washing the plutonium metal composition in water. However, it is preferable to remove this coating :by immersing the plutonium metal composition in a concentrated solution of ammonium nitrate in phosphoric acid, which immersion also improves the etch. (See applicants copending patent application Ser. No. 23,864 filed Apr. 28, 1948, now US. Patent No. 2,915,387.) The etched plutonium metal composition may then be used for any desired purpose or it may conveniently be coated with a smooth, uniform coating.

It is desirable before etching to clean the surface of the plutonium metal composition thoroughly. This may be accomplished, for example, by degreasing the plutonium metal composition in a volatile solvent such as acetone and then rendering the cleaned plutonium metal composition anodic in a soluble cyanide as described in copending application S.N. 34,352, filed June 21, 1948, now abandoned.

The invention may best be understood by considering its presently preferred embodiment given in the following example. This example is given for the purpose of illustration only, and not for the purpose of limiting the spirit or scope of the invention.

3,3 75,149 Patented Mar. 26, 1968 EXAMPLE A plutonium cube /2 inch on a side is degreased by immersion in acetone. The plutonium is then rendered anodic for five minutes in a solution containing grams of sodium cyanide per liter of electrolyte.

After the preliminary cleaning step the plutonium is etched by immersion in concentrated orthophosphoric acid for five minutes. The plutonium is then immersed in a solution containing 600 grams of ammonium nitrate per liter of orthophosphoric acid. The plutonium is washed successively in water and acetone, and is then permitted to dry.

The plutonium is placed in a container and is further cleaned and out-gassed by evacuating the container and I heating it by means of an external induction heating coil to about 200 C. The temperature is then reduced to C., the vacuum pumps are turned otf, and nickel carbonyl is admitted at a pressure of about 15 centimeters. When the pressure reaches the proper value in the previously calibrated system. the gas is pumped otf. Helium is admitted and the finished piece is cooled and removed from the system. The coating is smooth, uniform, and free of all pore holes.

It may be readily seen that the method of this invention as presented in this example may be varied in many ways. For example, the initial cleaning step may comprise machining the plutonium metal composition just prior to etching. The time the plutonium metal composition is immersed in the concentrated orthophosphoric acid may also be varied. Inasmuch as orthophosphoric acid is not very volatile, it may be conveniently used in a closed system and, of course, the dipping may be by remote control.

A method of the invention can be applied not only to plutonium metals but also to plutonium alloys or plutonium metal compacts containing a major proportion by weight of plutonium.

The additional etching step of immersing the plutonium metal composition in a concentrated solution of ammonium nitrate in or-thophosphoric acid is not necessary, but it is preferable.

After the plutonium metal composition has been etched, it may be used immediately or it may be plated. Instead of being plated with nickel by the decomposition of nickel carbonyl, it may be plated with other metals by the decomposition of other metal compounds or it may be electroplated or sprayed with metal. Various combinations of these or other plating methods may also be used.

It is to be understood that when plutonium metal compositions are mentioned in the specification or in the claims, pure plutonium, plutonium alloys, or plutonium metal compacts containing a major proportion by weight plutonium are intended.

Because there are many possible embodiments of the method of this invention, it is to -be understood that it is not to be limited except as indicated in the appended claims.

What is claimed is:

1. The method of etching the surface of a plutonium metal composition which comprises immersing a plutonium metal composition in concentrated orthophosphoric acid for a period of time sufficient to etch the plutonium surface and then withdrawing the plutonium metal composition from said acid.

2. The method of etching the surface of a plutonium metal composition which comprises immersing a plutonium metal composition in concentrated orthophosphoric acid for about five minutes until the plutonium surface is etched, and withdrawing the plutonium metal composition from said acid.

3. A method of preparing the surface of a plutonium metal composition to accept a uniform coating material which comprises immersing a plutonium metal composition in concentrated orthophosphoric acid for a period of time suflicient to etch the plutonium surface, immersing said etched plutonium metal composition in a solution containing a strong concentration of ammonium nitrate in concentrated orthophosphoric acid, and removing said plutonium metal composition from said solution.

4. A method of preparing the surface of a plutonium metal composition to accept a uniform metal coating material which comprises immersing a plutonium metal composition in concentrated orthophosphoric acid for about five minutes until the plutonium surface is etched, and then immersing the plutonium metal composition in a solution containing a strong concentration of ammonium nitrate in concentrated orthophosphoric acid, and withdrawing the plutonium met-al composition from said solution.

References Cited UNITED STATES PATENTS 1,329,573 2/1920 Allen 252142 X 1,715,695 6/1929 Cole 252-142 X 2,070,487 2/1937 Lutz 252142 X OTHER REFERENCES JACOB H. STEINBERG, Primary Examiner.

5 JOHN R. SPEC'K, WILLIS G. WILES, Examiners.

C. D. QUAREORTH, ROGER L. CAMPBELL,

Assistant Examiners. 

1. THE METHOD OF ETCHING THE SURFACE OF A PLUTONIUM METAL COMPOSITION WHICH COMPRISES IMMERSING A PLUTONIUM METAL COMPOSITION IN CONCENTRATED ORTHOPHOSPHORIC ACID FOR A PERIOD OF TIME SUFFICIENT TO ETCH THE PLUTONIUM SURFACE AND THEN WITHDRAWING THE PLUTONIUM METAL COMPOSITION FROM SAID ACID. 